and right-half (when swapping) so as to allow a wider
range of options
+<img src="/openpower/sv/grevlut2x2.jpg" width=700 />
+
grevlut should be arranged so as to produce the constants
needed to put into bext (bitextract) so as in turn to
be able to emulate x86 pmovmask instructions <https://www.felixcloutier.com/x86/pmovmskb>
-<img src="/openpower/sv/grevlut2x2.jpg" width=700 />
+Better diagram showing the correct ordering of shamt.
+
+<img src="/openpower/sv/grevlut.png" width=700 />
```
lut2(imm, a, b):